1. Field of the Invention
The present invention relates to novel aspartyl dipeptide ester derivatives, and sweeteners and products such as foods which contain the same as an active ingredient.
2. Description of the Related Art
In recent years, as eating habits and products eaten have changed, fatness caused by excessive intake of sugar and diseases caused by fatness have been of significant concern. Accordingly, the development of a low-calory sweetener that replaces sugar has been in demand. A sweetener that has been widely used is aspartame which is excellent in regarding both safety and taste properties. However, there are some problems of the aspartame with regard to its stability. WO 94/11391 states that derivatives in which an alkyl group is introduced on the amino group of aspartic acid constituting aspartame markedly improves sweetening potency and also results in a slight improvement in the stability of the compound. The best compound described in this document is N-[N-(3,3-dimethylbutyl)-L-xcex1--aspartyl]-L-phenylalanine 1-methyl ester having a 3,3-dimethylbutyl group as an alkyl group which has a sweetening potency 10,000 times that of sugar.
Aspartame derivatives having 20 different substituents other than the 3,3-dimethylbutyl group are also disclosed where the derivatives have sweetening potencies less than 2,500 times that of sugar. Derivatives having a 3-(substituted phenyl)propyl group as an alkyl group are also shown. However, it is reported that the sweetening potency of N-[N-(3-phenylpropyl)-L-xcex1-aspartyl]-L-phenylalanine 1-methyl ester is 1,500 times that of sugar and that of N-[N-[3-(3-methoxy-4-hydroxyphenyl)propyl]-L-xcex1-aspartyl]-L-phenylalanine 1-methyl ester is 2,500 times that of sugar. These sweetening potencies are far less than that of N-[N-(3,3-dimethylbutyl)-L-xcex1-aspartyl)-L-phenylalanine 1-methyl ester, 10,000 times. Furthermore, N-[N-(3,3-dimethylbutyl)-L-xcex1-aspartyll-L-tyrosine 1-methyl ester is stated therein as an example of the derivatives in which L-phenylalanine methyl ester is replaced by the other amino acid ester, and it is reported that its sweetening potency is 4,000 times that of sugar.
Under these circumstances, development of a low-calory sweetener having fine sweetening potency is in demand.
A problem to be solved by the present invention is to provide novel aspartyl dipeptide ester derivatives which are excellent in the safety and which have sweetening potency equal to or higher than that of the N-[N-(3,3-dimethylbutyl)-L-xcex1-aspartyl]-L-phenylalanine 1-methyl ester, a low-calory sweetener containing the same as an active ingredient and products, such as foods, containing the sweetener.
In order to solve the problem, the present inventors have synthesized several aspartame derivatives in which various 3-(substituted phenyl) propyl group are introduced onto a nitrogen atom of aspartic acid constituting the aspartame derivatives, wherein a moiety of L-Phenylalanine methyl ester in the aspartame is replaced by another amino acid ester, through reductive alkylation by use of cinnamaldehydes having various substituent and which are readily available, or 3-phenylpropionaldehyde having various substituent that can easily derived therefrom as precursor aldehyde, and have examined the sweetening potency of them. The Inventors have discovered that with respect to the sweetening potency, the novel compounds have a far higher sweetening potency than N-[N-(3,3-dimethylbutyl)-L-xcex1-aspartyl]-L-tyrosine 1-methyl ester which is reported to have the sweetening potency of 4,000 times that of sugar in WO 94/11391 and N-(N-(3,3-dimethylbutyl)-L-xcex1-aspartyl]-L-phenylalanine 1-methyl ester which is reported therein to have the potency of 10,000 times that of sugar. Particularly, the compounds represented by the following formula (1) are excellent as sweeteners. These findings have led to the completion of the present invention.
The present invention is directed to novel aspartyl dipeptide ester derivatives (including the salts thereof) represented by the following formula (1): 
wherein;
R1, R2, R3, R4 and R5, independently from each other, each represents a substituent selected from a hydrogen atom (H), a hydroxyl group (OH), an alkoxy group having from 1 to 3 carbon atoms (OCH3,OCH2CH3, OCH2CH2CH3, etc.), an alkyl group having from 1 to 3 carbon atoms (CH3, CH2CH3, CH2CH2CH3, etc.) and a hydroxyalkyloxy group having 2 or 3 carbon atoms (O(CH2)2OH, OCH2CH(OH)CH3, etc.), or R1 and R2, or R2 and R3 together form a methylenedioxy group (OCH2O) wherein R4, R5, and R1 or R3 which does not form the methylenedioxy group, independently from each other, each represents any substituent as mentioned above designated for the R1, R3, R4 and R5, respectively; R6 represents a substituent selected from a hydrogen atom, a benzyl group (CH2C6H5), a p-hydroxybenzyl group (CH2C6H4-p-OH), a cyclohexylmethyl group (CH2C6H11), a phenyl group (C6H5), a cyclohexyl group (C6H11), a phenylethyl group (CH2CH2C6H5) and a cyclohexylethyl group (CH2CH2C6H11); R7 represents a substituent selected from a hydrogen atom, a methyl group (CH3), an ethyl group (CH2CH3), and an isopropyl group (CH(CH3)2); R8 represents a substituent selected from a methyl group, an ethyl group, an isopropyl group, a n-propyl group (CH2CH2CH3) and a t-butyl group (C(CH3)3);
provided the derivatives in which R6 represents a benzyl group and R7 represents a hydrogen atom at the same time, and the derivatives in which R8 represents a p-hydroxybenzyl group and R7 represents a hydrogen atom at the same time are excluded.
The novel aspartyl dipeptide ester derivatives of the present invention include the compounds represented by the above formula (1) and the salts thereof.
Preferred in the novel aspartyl dipeptide ester derivatives is the L-isomer of aspartic acid, whereas other amino acids may be L- or D-isomer forms.
In the compounds described above of the present invention, the following inventions are included as the embodiments for the preferable compounds:
(A) The compounds of formula (1) wherein R7 is a substituent selected from a methyl group, an ethyl group and an isopropyl group; R1, R2, R3, R4 and R5, independently from each other, each is a substituent selected from a hydrogen atom, a hydroxyl group, an alkoxy group having from 1 to 3 carbon atoms, an alkyl group having from 1 to 3 carbon atoms and a hydroxyalkyloxy group having 2 or 3 carbon atoms (for example, O(CH)2OH, OCH2CH (OH)CH3, etc.), or R1 and R2, or R2 and R3 together form a methylenedioxy group (OCH2O) wherein R4, R5 and, R1 or R3 which does not form the methylenedioxy group, independently from each other, each is a substituent selected from any substituent as mentioned above designated or exemplified for the R1, R3, R4 and R5; R6 is a substituent selected from a hydrogen atom, a benzyl group, p-hydroxybenzyl group, a cyclohexylmethyl group, a phenyl group, a cyclohexyl group, a phenylethyl group and a cyclohexylethyl group; R8 is a substituent selected from a methyl group, an ethyl group, an isopropyl group, a n-propyl group and a t-butyl group.
(B) The compounds of formula (1) wherein R6 is a substituent selected from a hydrogen atom, a cyclohexylmethyl group, a phenyl group, a cyclohexyl group, a phenylethyl group and a cyclohexylethyl group; R1, R2, R3, R4 and R5, independently from each other, each is a substituent selected from a hydrogen atom, a hydroxyl group, an alkoxy group having from 1 to 3 carbon atoms, an alkyl group having from 1 to 3 carbon atoms and a hydroxyalkyloxy group having 2 or 3 carbon atoms (for example, O(CH2)2OH, OCH2CH(OH)CH3, etc.), or R1 and R2, or R2 and R3 together form a methylenedioxy group (OCH2O) wherein R4, R5 and, R1 or R3 which does not form the methylenedioxy group, independently from each other, each is a substituent selected from any substituent as mentioned above designated or exemplified for the R1, R3, R4 and R5, respectively; R7 is a substituent selected from a hydrogen atom, a methyl group, an ethyl group and an isopropyl group; R8 is a substituent selected from a methyl group, an ethyl group, an isopropyl group, a n-propyl group and a t-butyl group.
(C) The compounds of formula (1) wherein R2 is a hydroxyl group, R3 is a methoxy group, R1, R4 and R5 are hydrogen atoms, R6 is a benzyl group, and R7 and R8 are methyl groups.
(D) The compounds of formula (1) wherein R2 is a methoxy group, R3 is a hydroxyl group, R1, R4 and R5 are hydrogen atoms, R6 is a benzyl groups and R7 and R8 are methyl groups.
(C) The compounds of formula (1) wherein R2 is a hydroxyl group, R3 is a methoxy group, R1, R4, R5 and R7 are hydrogen atoms, R6 is a cyclohexylmethyl group, and R8 is a methyl group.
(D) The compounds of formula (1) wherein R2 is a methoxy group, R3 is a hydroxyl group, R1, R4, R5 and R7 are hydrogen atoms, R6 is a cyclohexylmethyl group, and R8 is a methyl group.
(E) The compounds of formula (1) wherein R2 is a hydroxyl group, R3 is a methoxy group, R1, R4, R5 and R7 are hydrogen atoms, R6 is a phenyl group, and R8 is a methyl group.
(F) The compounds of formula (1) wherein R2 is a methoxy group, R3 is a hydroxyl group, R1, R4, R5 and R7 are hydrogen atoms, R6 is a phenyl group, and R8 is a methyl group.
(G) The compounds of formula (1) wherein R2 is a hydroxyl group, R3 is a methoxy group, R1, R4, R5 and R7 are hydrogen atoms, R6 is a 2-phenylethyl group, and R8 is a methyl group.
(H) The compounds of formula (1) wherein R2 is a methoxy group, R3 is a hydroxyl group, R1, R4, R5 and R7 are hydrogen atoms, R6 is a 2-phenylethyl group, and R8 is a methyl group.
(I) The compounds of formula (1) wherein R2 is a hydroxyl group, R3 is a methoxy group, R1, R4, R5 and R6 are hydrogen atoms, R7 is a methyl group, and R8 is a n-propyl group.
(J) The compounds of formula (1) wherein R2 is a methoxy group, R3 is a hydroxyl group, R1, R4, R5 and R6 are hydrogen atoms, R7 is a methyl group, and R8 is a n-propyl group.
(K) The compounds of formula (1) wherein R1 is a hydroxyl group, R3 is a methoxy group, R2, R4, and R are hydrogen atoms, R6 is a benzyl group, and R7 and R8 are methyl groups.
(L) The compounds of formula (1) wherein R2 and R8 are methyl groups, R3 is a hydroxyl group, R1, R4, R5 and R7 are hydrogen atoms, and R6 is a cyclohexylmethyl group.
(M) The compounds of formula (1) wherein R1 is a hydroxyl group, R3 and R8 are methyl groups, R2, R4, R5 and R7 are hydrogen atoms, and R6 is a cyclohexylmethyl group.
The present invention also provides sweeteners containing at least one of the above disclosed compounds as an active ingredient. The sweeteners may also contain a carrier or bulking agent.
The present invention also provides foods or other products requiring added sweetness.
The present invention also provides methods for imparting sweetness by incorporating or giving (adding, mixing, or the like) at least one of the compounds of formula (1) to products requiring the sweetness such as foods, drinks (beverages), pharmaceutical products, oral hygiene products and the like.
In a method for producing compounds of the general formula (1) described above, an aldehyde represented by the following general formula (2) or (3) is reacted with an aspartame derivative represented by the following formula (4) under reductive alkylation conditions. 
wherein R1, R2, R3, R4 and R5 have the same meanings as those in the R1, R2, R3, R4 and R5 mentioned in the above formula (1) for the derivatives in the present invention. 
wherein R6, R7 and R8 have the same meanings as those in the R6, R7 and R8 mentioned in the above general formula (1) for the derivatives in the present invention; R9 represents a substituent selected from a hydrogen atom and a substituent which is convertible to a hydrogen atom under reductive alkylation conditions; and R10 represents a substituent selected from a hydrogen atom and a substituent which can be used for protecting a carboxyl group, such as a benzyl group and a t-butyl group.
Said method comprises the step of reacting under any reductive alkylation conditions, and may further comprise additional steps, for example, to obtain the compound of general formula (1) such steps for removing protective groups and forming salts after the step of reacting under the reductive alkylation conditions, if necessary.
As said substituent which is convertible to a hydrogen atom under the reductive alkylation conditions, any substituent which meets said condition can be selected from known substituents therefor, for example, a benzyloxycarbonyl group, a p-nitrobenzyloxycarbonyl group, a p-methoxybenzyloxycarbonyl group, a benzyl group, and dibenzyl groups (N,N-dibenzyl groups). The reductive alkylation conditions which are appropriate include those which are known in the art or those developed at a later time, for example, using a metal hydride.
In the above-disclosed method of producing compounds of formula (10 wherein one or more hydroxyl groups in the aldehyde represented by the above general formula (2) or (3) are protected by any appropriate protecting groups (for example, a benzyl group) in the case of the aldehyde having one or more hydroxyl groups.
Examples of salts of the present compounds include; alkaline metal salts such as sodium and potassium; alkaline earth metal salts such as calcium and magnesium; ammonium salts with ammonia; amino acid salts such as lysine and arginine; inorganic acid salts such as hydrochloric acid and sulfuric acid; and organic acid salts such as citric acid and acetic acid; and salts of other sweeteners such as saccharin, acesulfame, cyclamic acid, glycyrrhizic acid. These salts are included in the present derivatives.
The aspartyl dipeptide ester derivatives of the present invention can easily be formed by reductively alkylating aspartame derivatives, wherein a moiety of L-phenylalanine methyl ester in the aspartame is replaced by another amino acid ester, with cinnamaldehydes having various substituents and a reducing agent (for example, hydrogen/palladium carbon catalyst). Alternatively, the derivatives can be formed by subjecting aspartame derivatives (for example, xcex2-O-benzyl(xcex1-L-aspartyl-L-amino acid methyl ester) having a protective group in a carboxylic acid in the xcex2-position which derivatives can be obtained by the usual peptide synthesis method (Izumiya et al., Basis of Peptide Synthesis and Experiments Thereof, Maruzen, published Jan. 20, 1985, the contents of which are incorporated by reference) to reductive alkylation with cinnamaldehydes having various substituents and a reducing agent (for example, NaB(OAc)3H) (A. F. Abdel-Magid et al., Tetrahedron Letters, 31, 5595 (1990), the contents of which are incorporated by reference), and then removing the protective group. However, the method of forming the compounds of the present invention is not limited thereto. 3-Phenylpropionaldehydes having various substituents or acetal derivatives thereof can be used as precursor aldehydes in the reductive alkylation instead of cinnamaldehydes having various substituents.
As a result of sensory evaluations, the present compounds and the salts thereof were found to have a strong sweetening potency and have sensory (organoleptic) properties similar to that of sugar. For example, the sweetening potency of N-[N-[3-(3-hydroxy-4-methoxyphenyl)propyl]-L-xcex1-aspartyl]-L-(xcex1-methyl)phenylalanine 1-methyl ester was approximately 18,000 times (relative to sugar), that of N-[N-[3-(3-methoxy-4-hydroxyphenyl)propyl]-L-xcex1-aspartyl]-L-(xcex1-methy)phenylalanine 1-methyl ester was approximately 18,000 times (relative to sugar), that of N-[N-[3-(3-hydroxy-4-methoxyphenyl)propyl]-L-xcex1-aspartyl]-3-cyclohexyl-L-alanine 1-methyl ester was approximately 25,000 times (relative to sugar), that of N-[N-[3-(3-methoxy-4-hydroxyphenyl)propyl]-L-xcex1-aspartyl]-3-cyclohexyl-L-alanine 1-methyl ester was approximately 25,000 times (relative to sugar), that of N-[N-[3-(3-methyl-4-hydroxyphenyl)propyl]-L-(xcex1-aspartyl]-3-cyclohexyl-L-alanine 1-methyl ester was approximately 40,000 times (relative to sugar).
With respect to the aspartyl dipeptide derivatives (represented by the following general formula (5)) formed, the structures and the results of the sensory evaluation are shown in Table 1. 
As understood from the results of Table 1, the novel derivatives in the present invention have excellent sweetening potency.
When the derivatives (including compounds in the present invention and the salts thereof) of the present invention are used as sweeteners, these may of course be used in combination with other sweeteners as desired or needed.
When the derivatives of the present invention are used as sweeteners, an appropriate carrier and/or an appropriate bulking agent may be used as required. For example, a carrier, a bulking agent or the like which is known in the art and so far used for the sweeteners is available. The appropriate carriers or bulking agent may be selected from polydextrose, starch, maltodextrines, cellulose, methylcellulose, carboxymethylcellulose and other cellulose derivatives, sodium alginate, pectins, gums, lactose, maltose, glucose, sucrose, leucine, glycerole, mannitol, sorbitol, xylitol, erythritol, and equivalents thereof.
The derivatives of the present invention can be used as sweeteners or ingredients therefor, and further as sweeteners for products such as foods and the like to which a sweetness has to be imparted, for example, confectionery, chewing gum, hygiene products, toiletries, cosmetics, pharmaceutical products and veterinary products for animals. Still further, they can be used as a form of products having sweetness including the derivatives of the present invention and they can be used in a method of imparting sweetness to the products requiring sweetness. The method therefor can be, known methods for example, conventional methods for using a sweetening ingredient for a sweetener in the sweeteners or the method of imparting sweetness.
Having generally described this invention, a further understanding can be obtained by reference to certain specific examples which are provided herein for purposes of illustration only, and are not intended to be limiting unless otherwise specified.
This application is based on International Application Serial No PCT/JP99/03050, filed Jun. 7, 1999 and Japanese Patent Application Serial No. 10-180204, filed on Jun. 26, 1998, each of which are incorporated herein by references in their entirety.